1. Field of the Invention
The present invention relates to electronic devices, and in particular, to reference voltage generators.
2. Description of Related Art
Noise on input pins of microprocessors continues to play an ever crucial role in more recent designs. Increased complexity of these systems leads to increased density of signals and this, combined with greater signaling speeds, produces larger system switching noise as well as cross-talk noise. Further, continued reduction of supply voltages also reduces noise-margins and a general degradation of overall system noise immunity. Cost pressures that contribute to a reduction in the number of layers and an increased variability of line parameters in printed circuit boards (PCBs) produce an overall reduction in signal quality of even the choicest routes. In many designs, signals that are more critical in terms of noise and speed receive the shortest and choicest routes while signals that are slower and somewhat less timing critical end up with fairly lengthy and not the most desirable routes. In such designs, these types of signals end up with the worst level of noise and signal integrity. To make matters worse, backwards design compatibility to legacy systems forces even newer designs to stick to design requirements that were deemed marginal to begin with. All of these factors tend to force silicon designers to continually improve their receiver noise immunity on newer designs. This, by itself, is a challenge as reduced supply voltages continually degrade noise rejection of input receivers.
One technique to improve the noise margin of input receivers is the use of hysteresis. Hysteresis is a technique that improves noise margin by shifting the switching point of a given receiver up for a rising edge input and down for a downward switching signal. The transfer characteristic of a receiver with hysteresis is shown in FIG. 1. In many designs it is sufficient to just build some hysteresis into the receiver without actually bounding the actual design by requiring some voltage limits on it. Thus, FIG. 1 shows a receiver with a minimum hysteresis above or below the mid-point of the input transition.
As shown in FIG. 2, in many applications just having minimum hysteresis does not suffice and the design is expected to incorporate a maximum limit of hysteresis for both the low-to-high (L-H) and the high-to-low (H-L) transitions. In this case, the design requirement is such that the input receiver transitions within the voltage bands shown in this FIG. 2 by the maximum and minimum voltages Vh− and Vh+. This constraint is important in systems where incoming signals do not switch rail to rail or even in systems where the incoming signal is expected to slow down considerably beyond a certain point of its transition. Furthermore, in many applications it is required that the receiver switches precisely at the threshold switching voltages Vh+ and a Vh− for signals that are very timing critical. A typical specification sheet for such input pins is shown below in Table I below with minimum and maximum Vh+/Vh− voltages.
TABLE IVh+ input LH(VCC + VHYS_MIN)/(VCC + VHYS_MAX)/2.0threshold voltage2.0Vh− input HL(VCC − VHYS_MAX)/(VCC − VHYS_MIN)/2.0threshold voltage2.0With these specification, (VHYS_MAX-VHYS_MIN)/2.0 is the maximum range of a hysteresis variation window. In summary, the invariability of the voltages Vh+ and Vh− is critical in many applications.
There are number of methods in the prior art to incorporate hysteresis into an input receiver for a microprocessor pin. As shown in FIG. 3, typically a hysteresis circuit 10 for an input receiver includes a reference voltage generator 12 which is controlled by an OUTPUT signal of a sensing amplifier 14. The sensing amplifier 14 is a comparator, which has a digital one (high level output voltage) or digital zero (low level output voltage). The transition from one level to another occurs at the value given by the reference voltage VREF. In other words, the sensing amplifier 14 is used to determine when a voltage of the INPUT signal goes above the threshold reference voltage VREF and thereafter produces a one output when that occurs.
If the output of the sensing amplifier 14 is low, the voltage of reference generator 12 is pulled up to the Vh+ value, as shown in FIG. 4. If the output is a high, the voltage of the reference generator 12 is pulled down to Vh− value (Vh− shown in FIG. 4, but not a corresponding input and output signals). In this manner, this hysteresis circuit 10 implements the characteristics shown in FIG. 2 by switching at the voltage Vh+ on a rising edge and the voltage Vh− on a falling edge. As mentioned earlier, many systems require very tight voltage bands around the voltages Vh+ and Vh−.
With reference to FIG. 5, there is shown an implementation of a prior art reference generator 12 for the hysteresis circuit 10. It includes three p-channel transistors 18, 20, and 22 and two n-channel transistors 24 and 26. The two n-channel transistors in may easily be replaced with just one. The variation of Vh+ or Vh− voltages for a given supply voltage Vcc is primarily determined by the process variation and temperature. The signal VREF_CTRL in FIG. 3 is either a digital one or zero depending on what the voltage VREF needs to be.